William IX (Occitan: Guilhèm de Peitieus; French: Guillaume de Poitiers) (22 October 1071 – 10 February 1126), called the Troubador, is the Duke of Aquitaine and Gascony and Count of Poitou (as William VII) between 1086 and his death.

He is also one of the leaders of the Crusade of 1101.

Though his political and military achievements have a certain historical importance, he is best known as the earliest troubadour — a vernacular lyric poet in the Occitan language — whose work survives.